With the rapid development of thin film transistor (TFT) display technology, display devices tend to be manufactured in increasingly larger dimensions. For larger-scaled display devices, problems such as extended charging duration and even incomplete charging that causes decreased gray level have emerged due to increased rows and columns of gate lines and data lines. As a result, it is required to increase ON-state current in the TFT. On the other hand, however, excessively larger OFF-state current in the TFT may degrade its capability of holding voltage and also increase power consumption, thus it also needs to increase a switch ratio of the TFT and hence to further improve a performance thereof.